Street sweeping captures nonpoint-source pollutants–but when, where, and how often to do it?

By J.P. Partland

"Could the answer to the ultimate survival of our Northwest salmon be high-efficiency sweeping?" Consultant Roger Sutherland, vice president of Pacific Water Resources in Beaverton, OR, poses this rhetorical question. Maybe sweepers can't do it alone, but they can make a difference, according to Sutherland. "What I'm trying to emphasize is that if it is as effective as we think it can be, it will be the most cost-effective BMP [best management practice] in an urbanized environment," he explains. "It's always cheaper when source control can be effective."

When the source is stormwater runoff, source control is a big issue that involves every manmade outpost in the country. Every paved surface that can get wet is a place to look for pollutants. "There is a certain redundancy in the phrase Œbacteria-impaired urban streams,'" notes Don Waye, senior water resources planner for the Northern Virginia Regional Commission. "Currently, bacteria standard violations are the nation's - and Virginia's - number-one cause of impairments." PM10 (particulate matter less than 10 microns in diameter) pollutants rest in the nooks and crannies of every roadway of the land. Hydrocarbons, pesticides, animal waste, antifreeze, and heavy metals, as well as silt and sand, rest inert on the road, waiting for a good rain to come along to ride the flume to a water source. Sutherland can qualify and quantify the PM10 on the road, but he puts it in more stark terms. "When the effective impervious area of a watershed reaches 10%, the net result is that the biological diversity starts to crash." It is easy to understand why when one realizes that over half of the pollutants targeted by the Clean Water Act (CWA) can be found in the mini-micron debris on the street.

Evolving Technology

Advances in street-sweeping technology make it possible to pick up enough of these nonpoint-source pollutants to help municipalities meet their CWA requirements. And for the municipalities that need to sell the public on aesthetic concerns in order to get sweepers on the road, the cleaners can sweep for both appearance and pollutants.

This wasn't always the case. Mechanical broom sweepers, designed to pick up standard road debris, such as mufflers, stones, and road kill, didn't do anything to help prevent toxic pollutants from running off into the streams and rivers. National Urban Runoff Program (NURP) studies in the 1980s found that the use of broom buggies did not result in measurable runoff improvement. In fact, they might have made water quality worse by taking up the surface dirt that the pollutants might have attached to and thus been prevented from going directly into runoff. The broom trucks probably also broke down pollutants into ever smaller sizes, making it easier for them to be transported by runoff into the water system. And the slightly newer broom sweepers could leave a toxic slurry from the water they used to reduce the dust clouds the machines themselves kicked up.

The latest generation of sweepers came about from necessity. The invention of regenerative air sweepers came when a road builder needed to clean debris out of the cracks and crevices of road surfaces to more permanently bond a new layer of pavement on top of the old one. The first attempt at cleaning involved blasting compressed air at the road surface. It is interesting to think that a similar necessity might drive more improvements in sweeper technology. Shoot air down at the road and the fine particles rise up, readying themselves to be caught.

The vacuum filter or small-micron-particulate sweeper came about from the railroads. Railroaders needed a way to pick up things (e.g., coal dust) that couldn't be mixed with water. To accomplish this, a filtration system that cleaned itself was developed.

Regenerative air sweepers have been around for the last 25 years, vacuum filter sweepers for less than 10, but both are turning the tide on sweeping. From coast to coast, municipalities interested in clean water find that sweepers are a vital part of their efforts to meet National Pollutant Discharge Elimination System (NPDES) standards.

Time appears to be both the promise and the obstacle for advancing the use of street sweeping as a pollution control method. On the one hand, mechanical broom sweepers are still in use. On the other hand, they'll eventually need replacing. Likewise, regulations continue to tighten. NPDES Phase II goes into effect in 2003, which will likely drive the technology further.

Where and When?

Venice, FL, received its NPDES permit in December 1994. Pat Collins, P.E., city engineer for Venice, notes, "A lot of us were already doing water-quality controls and BMPs for them by the time the permits came out. An NPDES permit was cheaper to implement here than it was in some other state that hadn't already been doing the work. We've been doing it since 1985. A lot of stormwater that runs off our site goes into treatment ponds and provides for flood control."

With street sweeping, Collins believes, "You want to look at the most bang for the buck. Do industrial areas first, then less intense land uses as your last priority. Here in Sarasota County, we do our major roadways first, then our arterial roads, then our residential streets later. We sweep major thoroughfares a couple of times a week. Neighborhoods, it varies - some things are done twice in the downtown area. The other areas are done once a week or once a month.

"We sweep the gutter," he continues. "Oils and greases and hydrocarbons have a natural affinity to sand particles, so we're picking up the sands and sediments that wash into the streets. The other things you also pick up are leaves and litter and that sort of thing that can cause blockage. By sweeping those, you help with flood control." Venice uses a Johnston 605, a truck-mounted broom vacuum system.

Venice addresses both aesthetics and water-quality issues in its sweeping program, but those aren't the only things in the city's favor. Good road surfaces are important too. Bad surfaces accumulate cracks that house pollutants, and the broken pavement becomes part of the debris that can get into the runoff. The curb and gutter help keep many natural elements, such as sediment and plant life, off the street and keep pollutants and trash on the street. As Nancy Breward, acting stormwater operations manager for San Antonio, TX, points out, "Curbs and gutters have defined the line for vegetation. For the noncurb streets, we have scrapers go first."

Another practice to minimize pollutants getting into the runoff involves carefully managing construction sites. Although sites 5 ac. or larger must have a stormwater plan under NPDES Phase I, the smaller ones that have been under the NPDES radar can have a great impact. Hossain Kazemi, environmental specialist supervisor with the San Francisco Regional Water Quality Control Board, looks at construction sites and cringes. "In the construction studies, they've shown that if they don't control runoff, the sediment discharge would be 10 to 100 times greater than an undisturbed area." His recommendation is to stop construction vehicles, including personal ones, from tracking mud. "Tell all the contractors not to put trucks on the mud. Throw down straw or scrap wood so people don't track mud. You have to educate and train everybody. If you do the right thing up front, you don't have to spend money on cleaning." He advocates stiff penalties of both fines and work stoppages if regulations aren't adhered to.

San Antonio takes another tack. According to Breward, the city accepts input from the community about where to send the sweepers. The city will heed requests to do extra sweeps near a construction site. "We also do neighborhood sweeps, where we go into a community and concentrate on providing special services for a two-week period," says Breward. It could involve daily sweeps for that fortnight. San Antonio uses Tymco regenerative air sweepers.

How Often Is Often Enough?

Don Waye has studied sweeping intervals. "This is highly variable and is usually based on resources. There is usually a seasonal component to most street sweeping programs, and climate also plays a role. Many sweeping programs are event-based, but these are usually programs that focus on aesthetics as opposed to water quality. Optimal frequency for water-quality benefits is the subject of many debates. Sutherland's studies go the farthest to answer the question of the optimal sweeping interval and show a clear association between water-quality benefits and sweeping frequency, but there is a point of diminishing returns. In most cases, biweekly is about right. However, the optimal sweeping program from a theoretical standpoint is one that completes its sweep immediately before each rainfall. Such a weather-dependent strategy would mean one thing for the arid Southwest and quite another for the rainy Northwest."

Sweeping intervals remain critical in terms of maximizing pickup of pollutants and debris. Some roads get cleaned twice a year, while others are cleaned daily. Figuring these numbers is a tricky business, depending on traffic volume, water volume, and climate. Funding certainly plays a part as well, but that isn't always discussed.

When sweeping for appearance's sake, it makes perfect sense to return to various streets on a regular basis, calculated on the volume of debris left on the road per day. Yet in terms of water quality, the ideal timing for sweeping isn't an interval at all. Small particles will hang out largely unseen and are not a danger until the rains come. If there is a trick, it's to accurately predict rain so that the streets are all swept moments before the rain hits. Since this kind of prediction and speed is nigh impossible, it is easy to see why many look at biweekly sweeping as being a limit before returns start to diminish.

Guessing rainfall in many parts of the country is often difficult. But in dry climates - such as San Antonio, currently in the middle of a several-year drought - it makes sense to sweep the quiet streets just twice a year. San Antonio has played a leading role in stormwater treatment, primarily because of the city's famed - and tourist-friendly - downtown Riverwalk. Keeping the San Antonio River clean-looking and healthy is an economic benefit to the city, one that residents enthusiastically support. They can afford to sweep their downtown streets 360 times a year. Breward estimates that the city also sweeps the arterial and collector streets four times a year. These intervals will probably not need to be adjusted unless a serious climate shift occurs. Even when the rains hit San Antonio, they usually come frequently over a short period of time and then disappear for a much longer period.

Wet, humid, temperate, and cold climates have different needs and accordingly require different sweeping strategies. Some of the most prominent experts on stormwater and sweeping, including Sutherland and Mark Blosser of Olympia, WA, are from the Pacific Northwest. Here, too, there is steady support for implementing environmentally friendly solutions. Sutherland favors limiting sweeping during the rainy season. "Dissolution is a solution" is a koan he repeats. He means that with the greater water volume in the rivers during rainy season, the particles and pollutants make up a smaller percentage of stormwater runoff and become a smaller percentage of the river's toxic elements, thus rendering them "safer." The flip side is that runoff in the dry seasons has a higher concentration of pollutants.

Keep in mind that rain and moisture are two different things. Most sweepers can work on damp roadways. Yet even Blosser, a water resources engineer for the City of Olympia and a proponent of sweeping, says flat out, "It doesn't work in the rain. You shouldn't be sweeping for pollution control when it's wet. If there's water running down the gutter line, you wouldn't use it. We have dry pavement three-fourths of the time. You can still use [the sweepers] when it's damp. It's dry here from May through October. Pollutants build up on the streets then."

An issue Blosser and the City of Olympia recently faced concerned whether to grant a variance allowing the expansion of the Capital Mall to forgo passive, in-ground treatment systems in favor of a sweeper - specifically, the type known as a "particulate management street cleaner," the EV1 from Schwarze Industries of Huntsville, AL. If the city approved the variance, the mall would give the city one sweeper and $50,000 a year to cover the cost of an operator and repairs.

Blosser determined that the EV1 would remove approximately 36 times more pollutants from stormwater runoff than the in-ground filtering system specified for the mall, and it would do so for significantly less money. He estimated the in-ground system would remove just under 20,000 lb. of solids per year, while the EV1 would remove about 720,000 lb. At the time, Olympia owned only mechanical broom sweepers. "The EV1 would complement the city's existing sweeping operation," Blosser said in his report. "It would sweep mostly high-traffic arterial streets outside of downtown Olympia, removing the fine particles that contain the pollutants. This reduces stormwater contamination, benefiting streams and Puget Sound, and improves air quality, benefiting everyone." The downtown area already has a stormwater plan that utilizes a wastewater treatment facility.

Comparing costs, Blosser found each machine costs $300,000 plus $50,000 annual maintenance costs, versus $27 million capital investment in creating the in-ground system plus $1.8 million in annual maintenance.

The Blosser report discusses two interesting tests. One is a controlled-setting test in California in which the EV1 picked up more than 99% of "a fixed quantity of mixed particles." The other is a sweep-off in Virginia between the EV1 and a mechanical broom sweeper. "The machines swept two sides of the same road, then switched sides and reswept the opposite sides. The EV picked up 540 pounds more on the first pass - 2,700 versus 2,160 - and 870 pounds more on the resweep - 1,080 versus 210." That included more than four times the total phosphorous, and there were vastly higher metal concentrations found inside the EV after the test. Blosser concluded that there should be a reduction of 30-40% of the pollutant loadings from the streets, even with use during only six months of the year. Blosser also found that most reported drawbacks concerned a lack of training and mechanical support. As of this writing, Olympia is waiting for the mall to decide whether or not it's ready to go ahead with the expansion.

Sutherland is also a big fan of the EV1. "There isn't another competitor, in my mind, that can do the job of the EV sweeper. It makes it difficult for folks in the public works business because it makes them nervous. I have a phrase to convince them of its effectiveness: high-efficiency sweeper." He even believes that Schwarze and the EV1's developer, EnviroWhirl (purchased by Schwarze in 1997), don't do enough to market the product. When looking at the drawbacks, Sutherland finds a culprit: "It can't travel [at speeds] greater than 25 miles per hour. It's perceived as a big deal for sweeper operators because they don't like to go slowly between sweeps." It is apparent how the driving speed would limit effectiveness if the sweeping area is on the far side of a municipality and the yard is on the other.

Other limits for any kind of sweeping are traffic-volume issues. No one wants to hold up traffic, but the best places to sweep are the most traveled roads. Sutherland measured a highway in suburban Milwaukee, WI, that had a daily car volume of 250,000. Pollutants settle in all cracks, but the only place that really has a chance of being cleaned is the shoulder.

Sutherland's study in Livonia, MI, demonstrated that many have overestimated the runoff problem in residential areas. He measured the accumulation of street dirt over eight months, then he plugged the numbers into a model. For that area, he estimated that yearly runoff was 42 lb./ac. It led him to conclude, "For some areas, perhaps single-family residential, a regenerative air sweeper is a good choice. Ninety percent of the sweepers in America today are mechanical. If you won't buy the best, get a regenerative air machine."

Others agree, if only for financial reasons. Waye states, "Typical sweepers cost about $60,000 to $120,000, with mechanical air on the low end of this range and regenerative air on the high end. The EV sweepers typically cost $240,000 to $310,000 initially, but they are probably cheaper to maintain in the long run. I say Œprobably' because the oldest sweeper of this type was built in 1994, and not many have been built, so there's not much of a track record."

Fellow Virginian Stuart Finley of the Lake Barcroft Watershed Improvement District has a CWA Section 319 grant from the EPA to test BMPs in a small community situated above a lake. Although he doesn't do daily cleanings, he is in a unique position to test products and theories. He wrote about his findings in his annual report: "The best combination is to sweep first with the [Schwarze] A7000 and then follow with the EV2. This confirms our premise that an initial wet sweep followed by an EV2 sweep to pick up fines is the most cost effective. A beneficial community strategy [all italics his] is to use both machines and assign them according to [the] nature of problem and the weather with a tandem sweep by [the] A7000 followed by the EV2."

Since his work is of an experimental nature and has no bearing on implementation, Finley is frustrated by what he sees in local municipalities. "They're making no effort to sweep in [Fairfax County, Virginia]. Neither the county nor VDOT does it, because neither group is yielding. Fairfax cleans leaves and VDOT moves traffic. The proposal we've made would pick up 1,700 tons of road waste over a four-year period. That stuff inevitably ends up in the sewers and then the lake, then the lake gets dredged out at the community's expense."

The irony for Finley is that he sees funding as something that should be easy. "The street sweeping has a duplicate objective: You're making it look better, but you're also trying to improve water quality. So you are looking at two sources of funding for revenue: community development and water quality."

That takes us back to the salmon. It is a symbol of the Pacific Northwest: one that helps tourism and that represents clean water. Keeping the salmon alive and thriving demonstrates that we have started to master our waste. Capturing nonpoint-source pollutants where they rest on the road helps to slow down and possibly reverse the "biodiversity crash."    

J.P. Partland is based in New York, NY.

Sweeper Types

Mechanical: Usually broom-type sweepers designed to pick up debris, these are the least expensive available. About 90% of street sweepers currently in use in the United States are of this type. Models include Elgin Pelican (three-wheel) and Eagle (four-wheel) sweepers, Athey's Mobile three- and four-wheel models, and Schwarze M-series sweepers.

Regenerative Air: This type of sweeper blows air onto the road surface, causing fine particles and sediments within pavement crevices to rise, then vacuums them up. This type of sweeper has been available for more than two decades. Elgin's Crosswind J, Mobile's RA730 series, Schwarze's A-series, and all Tymco sweepers are of this type.

Vacuum Filter: Also called vacuum-assisted or small-micron-particulate sweepers. Two general types are available: wet and dry. The dry type combines a mechanical (broom-sweeping) process with a vacuum to capture small particles it stirs up. The wet type uses water for dust suppression. Scrubber-type machines apply water to the pavement so that fine particles are suspended, then vacuum up the mixture. Elgin's GeoVac and Whirlwind models are examples of the dry type, and Schwarze's EV-Series particulate management machines are a variant.

Tandem Sweeping: Two machines are used in this process: a first pass, usually by a mechanical sweeper, is followed by a second pass with a vacuum-type machine.